Machine powertrain systems are built in the modern age according to many different architectures. The powertrain system is generally understood as those components in a machine that produce rotational power and transmit it to ground-engaging propulsion elements, or a final drive coupled with the ground-engaging propulsion elements. A transmission coupled between an engine and the final drive is a core component of a machine powertrain. Mechanical transmissions in the nature of automatic transmissions having two, three, four or many more gear ratios are widely used. Manual transmissions remain in common usage at least for certain applications. As an alternative to, or sometimes in addition to mechanical gear sets, modern transmissions in off-highway or other utility machines are often hydraulic, or more particularly hydrostatic. In a hydrostatic transmission or “hystat” a hydraulic pump is provided and operates in conjunction with a hydraulic motor to provide a generally continuous range of transmission ratios produced by varying a displacement of the pump.
Still another increasingly popular powertrain system architecture is a so-called hybrid transmission that utilizes a combined hystat and mechanical design, where a hydrostatic transmission, referred to in this context as a variator, is coupled with components of a mechanical transmission. One or more variators, which can be driven from the engine geartrain or by way of some other technique, provide assistive torque to the transmission and can generally vary their input speed to the transmission across a relatively wide range. This enables the transmission to be operated by both a direct mechanical input from the engine and also an indirect mechanical input from the variators, enabling engine speed to be maintained relatively constant while varying the input speed provided to the transmission. A great many different variations on this general hybrid or split-path design have been proposed over the years. Despite many advances, there remains ample room for improvement, especially where multiple variators are used cooperatively, as even relatively minor mismatches in the variator characteristics or operation can end up reducing efficiency in the operation of the transmission. Commonly owned U.S. Pat. No. 8,500,587 to Hongliu et al. is directed to a system for providing hydraulic power in a transmission with multiple variators. A common input is geared to the inputs of the variators and a common output is geared to the outputs of the variators, tying the pump and motor of each variator to rotate at the same speed as the counterpart components of the other variator.